1. Field of the Invention
The present invention relates to a method and apparatus for non-contact measurement of relative displacement between two or more points, such as are used in various industrial fields to measure deformation of test pieces in material testing, to check spring properties, and to make other such measurements.
2. Description of the Prior Art
To measure deformation of a test piece in a material test procedure, to check the properties of a spring by measuring extension and/or compression deformation of the test piece and spring, and other such measurements are performed by gripping the test piece or spring with a test machine, subjecting the test piece or spring to deformation, and measuring the relative displacement between two points which are set a prescribed distance apart on the test piece. When the test piece is small, the amount of deformation is minute, the test takes place in a high temperature atmosphere and in other such cases in which a strain gage, extensometer or other such contact methods for measuring cannot be used, a microscope is used to measure the deformation. Specifically, the region of the test piece around the points of measurement is magnified in the field of view of the microscope, a video camera is used to take the magnified images, and the taken images are subjected to image processing to detect the relative displacement of the two points of measurement.
Conventionally this measurement has been performed using a microscope having one field of view, such as the microscope 51 shown in FIG. 6 (a). To use a single field of view microscope 51, marks 11 put on the test piece 7 to indicate the two points of measurement have to be located within the field of view 17 of the microscope. However, when the amount of measurement point displacement relative to the distance between the points of measurement is very small, it is difficult to measure the displacement, since it is impossible to magnify only the displacement of the two points. When the two points of measurement cannot be located within the field of view because of high magnifications, displacement can be measured by mounting the microscope on a translation stage and moving the microscope to measure displacement of the points of measurement before and after a load is applied to the test piece. When the displacement is very small, however, accurate measurements are difficult with this method owing to presence of such factors as the accuracy of the linear motion of the stage and positioning accuracy, and furthermore, it is also difficult to perform continuous measurements. Instead of using a single microscope, the arrangement shown in FIG. 7 (a) can be used in which two microscopes 51 are used, each trained on one of the two points of measurement 8 on the test piece 7. As shown in FIG. 7 (b), however, when there is not much distance between the two points of measurement 8, it is difficult to locate the points within the respective fields of view unless the microscopes are set very close together, since the approach between the two microscopes 51 gives rise to problems such as contact between the microscopes that often causes difficulties in practice.